The present invention relates to a method and apparatus for maintaining a fluid column height in a well. More specifically, the present invention relates to a method and apparatus for maintaining a fluid column height in an annulus between a first casing cemented in the well and a second casing being installed therethrough, thus maintaining hydrostatic pressure in the well.
In recent years, the drilling and completion of highly deviated wells, including horizontal wells, has increased appreciably. A horizontal well is one which includes one or more horizontal wellbore sections (i.e., wellbore sections drilled at an angle from a vertical of about 60xc2x0 or greater). The horizontal or deviated wellbore section or sections usually extend from a vertical or inclined wellbore section. The drilling of a horizontal well or section in a hydrocarbon producing zone allows more of the zone to be in direct contact with the wellbore which results in a higher displacement efficiency of the zone as a whole. In some xe2x80x9cextended reach wells,xe2x80x9d the horizontal wellbore sections frequently approach 90xc2x0 from vertical, and the horizontal wellbore sections are longer than the vertical sections. To complete horizontal wells, a casing string usually must be run into the horizontal wellbore section by sliding it through the wellbore. The drag forces exerted on the casing string can damage the joints at their threaded connections. As a result, expensive heavy casing joints with premium thread connections and torque shoulders have been utilized. The casing string can also become stuck as a result of differential pressures, which require the application of additional forces on the casing string. If sufficient additional forces cannot be applied, the stuck pipe may result in the loss of the well.
A number of techniques have been developed and used for decreasing the forces required to run casing strings in horizontal wells. For example, the wellbore drilling fluid has been replaced with a high-density fluid prior to running a casing string in a horizontal wellbore section to provide buoyant forces on the casing. In addition, a retrievable packer has been included in the casing string for the purpose of trapping a fluid lighter than the wellbore fluids between the packer and the end of the casing string. U.S. Pat. No. 4,986,361 dated Jan. 22, 1991, U.S. Pat. No. 5,117,915 dated Jun. 2, 1992, and U.S. Pat. No. 5,181,571 dated Jan. 26, 1993, all issued to Mueller et al., disclose apparatus for trapping air in the leading portion of a casing string to increase the buoyancy of the casing string in the drilling fluid contained in the wellbore. U.S. Pat. No. 5,829,526 (the ""526 patent) discloses an apparatus for trapping air in a first portion of the casing string causing the casing string to be buoyed up during placement by drilling fluid in the wellbore. The ""526 patent further discloses a selectively openable and releasable closed baffle assembly connected in the casing string for trapping a low density fluid, preferably air, in a second portion of the casing string, thereby causing it also to be buoyed up during placement of the casing string in the well by the drilling fluid in the wellbore.
The methods and apparatus described above have been successfully utilized for reducing casing string drag and eliminating the need for expensive heavy casing joints when placing a casing string in a horizontal wellbore. There are, however, potential risks associated with placement in the well of casing strings having buoyancy chambers therein. If the buoyancy chamber develops a leak or catastrophically fails and thus collapses, the fluid column in the wellbore will drop dramatically, as fluid in the wellbore moves to occupy the space originally filled by the buoyancy chamber. A loss of hydrostatic head will accompany the drop in fluid level. Such a loss in hydrostatic head can result in a severe well control situation and can cause loss of the control of the well, which is both dangerous and costly. Thus, there is a need for a method and apparatus for maintaining a fluid column in an annulus between a first casing string installed in a well and a second casing string being placed therethrough, when a potential for a drop in fluid level in the well exists.
The present invention provides an improved method and apparatus for maintaining a fluid column in an annulus between a first casing cemented in a well and a second casing being lowered therethrough. The apparatus of the present invention comprises a fluid column support, or fluid column seal disposed about and movable with the second casing. The fluid column support will engage the first casing cemented in the well. The fluid column support will allow flow upwardly in the annulus between the second casing and the first casing cemented in the well, but will prevent downward flow so that a column of fluid is supported in the annulus by the fluid column support. Because a fluid column is supported in the annulus, hydrostatic pressure can be maintained in the well in situations where a fluid level below the fluid column support drops in the well.
For example, the second casing may comprise a second casing being placed in a deviated well. The second casing may therefore include a float shoe at a lower end thereof and a float collar connected in the second casing above the float shoe. Connected casing joints between the float shoe and float collar may be filled with air or other compressible fluid to define a buoyancy chamber in the second casing.
The fluid column support is disposed about and movable with the second casing and will engage the first casing so that if the fluid level in the well below the fluid column support drops for any reason, such as for example a failure of the buoyancy chamber such that drilling fluid or other fluid in the well moves to occupy the space previously occupied by the buoyancy chamber, a column of fluid will be maintained in the annulus. There are preferably a plurality of fluid column supports spaced at intervals along the second casing. Because at least one of the plurality of seals disposed about the second casing will always be in engagement with the first casing as the second casing is being lowered therethrough to a desired location in the well, a column of fluid will always be supported in the annulus if the fluid level in the well below the engaged fluid column support drops for any reason, such as a failure of the buoyancy chamber.
The present invention thus provides a method and apparatus for maintaining a column of fluid in an annulus between a casing cemented in a wellbore and a second casing being lowered therethrough, and for retaining hydrostatic pressure in the well if the fluid level in the well below the supported column of fluid drops for any reason.